SCYON Abstract

Received on August 16 2002

Kinematic Parameters of Young Subsystems and the Galactic Rotation Curve

AuthorsM. V. Zabolotskikh, A. S. Rastorguev, A. K. Dambis
AffiliationSternberg Astronomical Institute, Universitetskii pr. 13, Moscow, 119992, Russia
Accepted byAstronomy Letters, 28, 454, 2002


We analyze the space velocities of blue supergiants, long-period Cepheids, and young open star clusters (OSCs), as well as the H I and H II radial-velocity fields by the maximum-likelihood method. The distance scales of the objects are matched both by comparing the first derivatives of the angular velocity Omega' determined separately from radial velocities and proper motions and by the statistical-parallax method. The former method yields a short distance scale (for Ro = 7.5 kpc, the assumed distances should be increased by 4%, whereas the latter method yields a long distance scale (for Ro = 8.5kpc, the assumed distances should be increased by 16%. We cannot choose between these two methods. Similarly, the distance scale of blue supergiants should be shortened by 9% and lengthened by 3%, respectively. The HII distance scale is matched with the distance scale of Cepheids and OSCs by comparing the derivatives Omega' determined for HII from radial velocities and for Cepheids and OSCs from space velocities. As a result, the distances to HII regions should be increased by 5% in the short distance scale. We constructed the Galactic rotation curve in the Galactocentric distance range 2-14 kpc from the radial velocities of all objects with allowance for the differences between the residual-velocity distributions. The axial ratio of the Cepheid+OSC velocity ellipsoid is well described by the Lindblad relation, while sigmau ~ sigmav for gas. The following rotation-curve parameters were obtained: Omegao = (27.5 ± 1.4) km/s/kpc and A = (17.1 ± 0.5) km/s/kpc for the short distance scale (Ro = 7.5 kpc); and Omegao = (26.6 ± 1.4) km/s/kpc and A = (15.4 ± 0.5) km/s/kpc for the long distance scale (Ro = 8.5 kpc). We propose a new method for determining the angular velocity Omegao from stellar radial velocities alone by using the Lindblad relation. Good agreement between the inferred Omegao and our calculations based on space velocities suggests that the Lindblad relation holds throughout the entire sample volume. Our analysis of the heliocentric velocities for samples of young-objects reveals noticeable streaming motions (with a velocity lag of ~7 km/s relative to the LSR), whereas a direct computation of the perturbation amplitudes in terms of linear density-wave theory yields a small amplitude for the tangential perturbations.